Pre-ます-Form VERB + (やすい/にくい) as “it is (easy/hard) to do VERB”
The い-adjectives やすい and にくい can be used after the Pre-ます-Form of a verb to express that it is easy or hard to perform that verb. This grammar construct can be used to express these varying degrees of simplicity and or difficulty in certain circumstances or in general; because やすい and にくい are い-adjectives, the past tense of these adjectives and this grammar construct would be やすかった and にくかった respectively.
Sono sensei wa totemo shinsetsu desukara hanashiyasui desu.
(Since that teacher is so nice, he/she is really easy to talk to.)
Kono toshokan wa sugoku shizuka dakara shuuchuu shiyasui.
(Since this library is very quiet, it’s easy to concentrate.)
Kyou no kougi wa wakari nikukatta.
(Today’s lecture was hard to understand.)